Retinoids are essential signaling molecules in animals and have become common chemo-preventitive dietary agents. The retinoid biosynthetic pathway begins with the cleavage of a carotenoid by a member of the Carotenoid Cleavage Dioxygenase (CCD) family of enzymes first discoverd in plants. In plants, CCDs participate in the biosynthesis of the phytohormone abscisic acid and a novel hormone that controls plant architecture. In animals the CCDs are responsible for the biosynthesis of retinoids, notably vitamin A, which is necessary for vision, and retinoic acid, which is critical to normal development. Currently, no high resolution structural information for any CCD exists. With the structure of one or more CCDs, a structurally guided approach towards enzyme and metabolic pathway engineering in plants could be done to allow for the control of plant architecture as well as increase nutritional quality. In animals, this approach could lead to a greater understanding of the production of the visual chromophore and to the signaling molecule, retinoic acid. The CCD family in the model organism Arabidopsis thaliana provides a collection of related genes for the systematic dissection of the structural and functional features of the mechanistically complex CCDs.